Wireless communications networks, such as the Third generation, 3G, wireless communications networks are wide area cellular telephone networks providing information services to end users via end user equipments. The information services may be e.g. text messaging, high-speed Internet access, video telephony and telephony. A user equipment can be e.g. a hand-held telephone, a card in a laptop computer, a personal digital assistant (PDA) and the like. The user equipment is varyingly known as a UE, a terminal equipment, a mobile station (MS), etc., depending on the standard.
A wireless communications network comprises a Core Network (CN) and several Radio Access Networks (RAN). RAN implements radio access technology allowing connectivity between user equipments and the CN via a base station such as e.g. a Node-B. The connection, i.e. the transmission path, between the user equipment and base station is called an up-link whereas the transmission path between the base station and the user equipment is called a down-link. The up-link and the down-link constitute the radio link. RAN further comprises Radio Network Controllers (RNC) providing control functionalities for one or more base stations. The RNC and its corresponding radio base stations are called the Radio Network Subsystem (RNS). There can be more than one RNS present in an RAN. Conceptually, it is situated between the user equipment and the CN. The main functions of the CN is to provide switching, routing and transmission of user traffic. CN further enables charging and network management functions.
Some of the communication protocols used in the RAN are the Medium Access Control (MAC) protocol and the Radio Link control (RLC) protocol. MAC and RLC are two protocols that, among other protocols, are included in the High Speed Packet Access (HSPA) collection of protocols for 3G wireless communications.
The wireless communications network of today offers a wide range of information services to the end users. The information services may be, as mentioned above e.g. text messaging, high-speed internet access, video telephony and telephony. The different services contains varying amounts of information, that is to say require different amounts of bandwidth when transmitted. Text messaging, such as Short Message Service (SMS) require only a small amount of bandwidth when transmitted whereas video telephony on the other hand require a considerable larger amount of bandwidth, to handle and synchronize both sound and vision. Typically, the radio link is the weak point of the transmission, providing the smallest amount of bandwidth. To get an as fast and effective transmission of information as possible, it is required that the data packets shall include as large part as possible of payload, and as small part as possible of overhead. Prior to the transmission over the radio link, the packets are segmented. Segmentation means that the incoming payload is divided into packet data units, of predefined and fixed sizes, these packet data units constituting the payload of the data packet to be sent. I.e. if the incoming payload is larger than the upper size limit of the data packet payload the incoming payload is divided into packet data units smaller or equal to the upper size limit.
However, the communication conditions influencing the bandwidth of the radio interface, i.e. the radio conditions, are not static. The radio conditions vary according to several aspects such as e.g. geographical surroundings, weather and number of simultaneous user equipments in the same radio cell and their service utilization. In order to comply with the requirements mentioned above of an effective transmission, a best effort approach is used. One way of compensating for poor radio conditions is to adapt segmentation in the sending node, i.e. reducing the maximum size of the packet data units when segmenting the incoming payload. This means that when the radio conditions are good, the sending node may transmit larger packet data units to the receiving node implying fast information transmission. However, if the radio conditions are bad, smaller packet data units requires to be transmitted, since trying to transmit too large packet data units may lead to poor communication efficiency such as unnecessary delays or even to lost information. The delays can e.g. be caused by excessive number of retransmissions whereas a cause, among others, of the lost information may be retransmission problems.
A disadvantage of having fixed and preconfigured packet data unit sizes is that there are too few/not enough of alternatives for packet data unit sizes to choose from when radio conditions are bad and segmentation of incoming payload is required. Either the packet data unit size is chosen too small which results in a slow information transmission caused by the large overhead of the data packets. On the other hand, trying to transmit too large packet data units, may lead to poor communication efficiency such as unnecessary delays or even to lost information. The delays can e.g. be caused by excessive number of retransmissions, such as e.g. HARQ (Hybrid Automatic Repeat request). HARQ is a functionality which, when the receiving node detects an erroneous packet data unit, sends a Negative Acknowledgement (NACK), to the sending node. The sending node responds, by retransmitting the requested packet data unit, to the receiving node. The HARQ process is repeated until either the packet data unit is successfully received by the receiver, or, a HARQ timer runs out. A cause, among others, of the lost information mentioned above, may be retransmission failure, such as e.g. HARQ failure. HARQ failure originates when the HARQ retransmission function fails, such as e.g. when the receiving node detects an erroneous packet data unit, sends a NACK to the sending node, but the sending node erroneously detects a positive Acknowledgement (ACK), and thus does not retransmit the requested packet data unit.